1. Field of the Invention
This invention provides a means for generating an objective, quantitative evaluation of road surfaces. The data generated are applicable to road design by providing quantitative data from which a numerical analysis of past, current, or proposed road projects can be made. These data also have significance for those responsible for road maintenance.
2. Prior Art
Accelerometers have been used by pavement engineers to determine pavement roughness. These instruments operate at a fixed speed (frequently 25 miles per hour) and vibrations from the road surface energize a recorder to accumulate the fluctuations. The summation of these fluctuations is expressed on a chart recording in two linear dimensions such as inches per mile with the inches being the summation of all the vertical deviations over the mile distance traveled. Although this type of instrument has some value, it is suitable primarily for relatively smooth surfaces. Because of the summary nature of the vertical measurement, all detail of the road surface is lost; a very rough road for a short distance followed by a smooth road could give the same reading as a medium rough road for the entire length, or if the right track would drop while the left track raised, the result could record as quite smooth. Obviously the more severe the variations in the road surface the less satisfactory the data generated by this instrument, and on unpaved roads such an instrument would be practically useless. One of the objects of this invention is to provide an instrument capable of measuring road surface roughness with deviations from less than one inch to those as large as the instrument carrier is capable of traversing; with a four-wheel drive vehicle, extremely large surface deviations are measurable. Another object of this invention is to provide an actual record of the surface deviations rather than an average over some traveled distance; thus each deviation within the sensitivity of the instrument is recorded as well as its distance from a known, selected point. With a chart from this invention, a complete, objective, quantitative record is obtained whereas the accelerometers described depended on a subjective evaluation of an observer to complement the recorded data; since the record contained merely a summation of all the vertical deviations, an independent evaluation of the road surface was a necessary supplement. For design or maintenance purposes, but particularly design, such a record is of minimal value because it is not keyed to any specific road section within the base distance traveled. A further advantage of this invention is that relatively unskilled operators can gather the data and an engineer or other skilled technician or professional can obtain a complete evaluation of the road largely from the data without having to spend inordinate amounts of time in the field.
A much more primitive method of determining surface roughness is determination by vertical measurements from an arbitrary plane; these measurements are obtained with a rod and level. A rutting index for unpaved roads has been developed from this method. The rutting index is vertical measurements in tenths of a foot from a mean elevation taken at one foot intervals across the road and divided by the width in feet of the measured road surface. This method suffers from the handicap of being very slow if a long stretch of road surface is involved. Additionally, it only gives quantitative data for the particular points measured and is void of such data for all points in between. This invention has the advantage of providing a continuous record of surface deviations over the entire length measured. It has the additional advantage of covering the desired surface at a much more rapid rate.
An analytical method for examining vehicle capability over extremely rough terrain was developed by Bekker of the Land Locomotion Laboratory, Ordnance Tank Automotive Command, Detroit, Michigan. This method uses a simple harmonic wave form with an amplitude close to the height of the vehicle that was theoretically analyzed. Analysis yielded "go" or "no go" conditions for each vehicle examined. This analysis does not correlate closely to actual conditions on a specific road section unless quantitative data of that road surface are first supplied, a function this invention provides. Additionally, the Bekker method provides no grading between passable and impassable.